Inhibition of human and mouse complement-dependent hemolytic activity by mouse fibronectin

Inhibitor(s) of the classical complement pathway in mouse serum limit the utility of mice as experimental models of neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system in which anti-aquaporin-4 (AQP4) autoantibodies (AQP4-IgG) cause damage to astrocytes by complement-dependent cytotoxicity (CDC). Various approaches have been attempted to produce NMO lesions in rodents, some involving genetically modified mice with altered immune cell function. Here, we found that mouse serum strongly inhibits complement from multiple species, preventing AQP4-IgG-dependent CDC. Effects of mouse serum on complement activation were tested in CDC assays in which AQP4-expressing cells were incubated with AQP4-IgG and complement from different species. Biochemical assays and mass spectrometry were used to characterize complement inhibitor(s) in mouse serum. Sera from different strains of mice produced almost no AQP4-IgG-dependent CDC compared with human, rat and guinea pig sera. Remarkably, addition of mouse serum prevented AQP4-IgG-dependent CDC caused by human, rat or guinea pig serum, with 50% inhibition at <5% mouse serum. Hemolysis assays indicated that the inhibitor(s) in mouse serum target the classical and not the alternative complement pathway. We found that the complement inhibitor(s) in mouse serum were contained in a serum fraction purified with protein-A resin; however, the inhibitor was not IgG as determined using serum from IgG-deficient mice. Mass spectrometry on the protein A-purified fraction produced several inhibitor candidates. The low intrinsic complement activity of mouse serum and the presence of complement inhibitor(s) limit the utility of mouse models to study disorders, such as NMO, involving the classical complement pathway.

A Unique Monoclonal Antibody Against Human Plasma Fibronectin, Which Recognizes an Epitope on the Surface of a Subpopulation of Polymorphonuclear Neutrophils

A new monoclonal antibody (MAb) specific to plasma fibronectin, named MO, which was produced by immunizing a mouse with fragments of fibronectin isolated from human plasma, was found to bind to the surface of a subpopulation of peripheral blood polymorphonuclear neutrophils (PMNs) in the presence of physiological concentration of soluble fibronectin. MAb MO had no reactivity with other blood cells such as lymphocytes, monocytes, or platelets. The percentage of the MAb MO-reactive subpopulation of PMNs from healthy donors varied from 0 to 60%. Ten previously established MAbs against fibronectin tested in this study together with MAb MO had no reactivity with the surface of PMNs at all, when tested simultaneously with MAb MO. These results indicate that MAb MO can be a unique and useful tool for analyzing the structure and function of fibronectin-especially the fibronectin expressed on the surface of PMNs.

Role of C-Reactive Protein in Atherogenesis

OBJECTIVE

Human C-reactive protein (CRP) was reported to accelerate atherosclerotic lesion development in male but not in female apolipoprotein E (apoE) knockout mice. Here, mice expressing rabbit CRP (rbCRP) were crossbred onto apoE knockout animals, and the effect on atherogenesis was studied.

METHODS AND RESULTS

Hemolytic complement activity could not be detected in apoE knockout mice. Furthermore, in contrast to human complement, neither rabbit nor human CRP complexed to modified low-density lipoprotein-activated murine complement. At 52 weeks, rbCRP levels were similar in male and female transgenic animals. Serum cholesterol levels were equivalent in female animals irrespective of rbCRP expression, whereas rbCRP-positive males had significantly higher serum cholesterol levels than the rbCRP-negative counterparts. All mice exhibited extensive atherosclerotic lesions, as studied en face, and no differences were noted between rbCRP-negative and rbCRP-positive animals. Atherosclerotic luminal obstruction of aortic arch and first-order neck branches did not differ significantly between rbCRP-positive and rbCRP-negative mice. There was no correlation between rbCRP levels and atherosclerotic lesion formation.

CONCLUSIONS

No marked effect of rbCRP on the formation of moderately advanced atherosclerotic lesions could be discerned in the apoE knockout mouse. Because of the oddities of the mouse complement system, however, this may not be a good model to investigate the role of CRP in human atherosclerosis.